1. Field of the Invention
This invention relates to magnetic recording disk files employing track following servo systems.
2. Description of Prior Art
The present invention relates to magnetic recording disk file information recording systems, and particularly to those systems employing continuous servo information for track following operations to enable high track densities with high lineal densities. Early magnetic recording disk files employed open loop positioning systems for controlling the movement of one or more magnetic heads to different concentric recording tracks on a magnetic disk or disks. With the demand for increased track density, closed loop servo systems were employed utilizing a separate disk surface dedicated to track following servo information, as first commercially exemplified by the IBM3330 disk file.
In some disk files, particularly in lower cost files such as the IBM 62PC, sector servo systems have been utilized, in which sectors of track-following information are interspersed around the disk surface between adjacent data areas. While such sector servo systems have the advantage that the servo information resides on the same disk as the data tracks on which track following is to take place, they have the disadvantage that they inherently reduce the area available for recording data.
It has long been recognized that a buried servo system, in which the servo information and data coexist in a disk, would be desirable. Even though such techniques have been known to be desirable, they have never been commerically exploited, apparently because of the degradation of the so-called buried servo or trackposition indicating signals or the expense of providing a multiple layer magnetic coating, one for positionindicating servo signals and another for the data signals.
An early teaching of superposed data and servo signals is found in U.S. Pat. No. 3,404,392. This patent teaches using a dual layer magnetic coating on a disk substrate. A lower layer has a higher retentivity and coercivity than the upper layer. A servo signal is first recorded in the lower layer for indicating track positions. Following such recording, data signals are recorded. Then both the servo and data signals are sensed. For each data recording operation, an erase gap precedes a read/write gap so that the data signals can be successfully recorded in the upper layer. During readback, not only are the data signals read back by a single gap, but also two sets of servo signals are simultaneously read back. These servo signals are linearly recorded sinewaves written in concentric circular tracks and exhibiting a low frequency. Adjacent servo tracks are written at different frequencies and each data track is defined as being superposed over one-half of an odd numbered servo track and one-half of an even numbered servo track. This patent teaches that the data signals and the servo signals must exist in the magnetic medium independently of the other. It also teaches that in a dual layered record medium, recording the data signals at a high frequency, at least three or four times the frequency of the servo signals in the lower layer, enables the data signals to be separated from the servo signals by filtering. The independence requirement was met in this patent by providing a record disk having a lower level layer with a higher coercivity than an upper data signal level layer. This arrangement allows the servo signals to remain undisturbed by subsequent writing and rewriting of data signals in the upper layer.
A later development is shown in the IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 1974, beginning at page 536. This shows a single magnetic coating on a substrate and teaches that the ratio of the frequency of the data signals to the servo signal should be 10:1 and that the servo signal can be successfully recorded with a head having sufficient ampere-turns capability.
U.S. Pat. No. 3,765,005 teaches using a clock-bias signal at the upper end of a data signal frequency spectrum. Also U.S. Pat. No. 3,821,798 shows a low frequency control signal recorded at a lower end portion of the data signal pass band.
A later reference showing buried servo is U.S. Pat. No. 3,956,769, which teaches the use of two separate gaps in a transducer; one for servo signals and one for data signals.
A still later reference is U.S. Pat. No. 4,318,141 in which a single-gap transducer simultaneously senses recorded control or servo signals and record data signals. A magnetic medium used is preferably a single layer magnetic medium. The head gap length and pole-tip face length allow different portions of the pole pieces in a magnetic transducer to be utilized during the simultaneous read and record operations. The servo signals are preferably deep and hard recorded into the layer, whereas the data signals are preferably pulse width limited and recorded adjacent the surface portion of the magnetic layer.